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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.overview.composition.spawn"></a><a class="link" href="spawn.html" title="Stackful Coroutines">Stackful Coroutines</a>
</h4></div></div></div>
<p>
          The <a class="link" href="../../reference/spawn.html" title="spawn"><code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code></a> function is a high-level wrapper
          for running stackful coroutines. It is based on the Boost.Coroutine library.
          The <code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code>
          function enables programs to implement asynchronous logic in a synchronous
          manner, as shown in the following example:
        </p>
<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">spawn</span><span class="special">(</span><span class="identifier">my_strand</span><span class="special">,</span> <span class="identifier">do_echo</span><span class="special">);</span>

<span class="comment">// ...</span>

<span class="keyword">void</span> <span class="identifier">do_echo</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">yield_context</span> <span class="identifier">yield</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">try</span>
  <span class="special">{</span>
    <span class="keyword">char</span> <span class="identifier">data</span><span class="special">[</span><span class="number">128</span><span class="special">];</span>
    <span class="keyword">for</span> <span class="special">(;;)</span>
    <span class="special">{</span>
      <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">length</span> <span class="special">=</span>
        <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
          <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">);</span>

      <span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_write</span><span class="special">(</span><span class="identifier">my_socket</span><span class="special">,</span>
          <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">length</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">);</span>
    <span class="special">}</span>
  <span class="special">}</span>
  <span class="keyword">catch</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span><span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span>
  <span class="special">{</span>
    <span class="comment">// ...</span>
  <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
          The first argument to <code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code> may be a <a class="link" href="../../reference/io_context__strand.html" title="io_context::strand"><code class="computeroutput"><span class="identifier">strand</span></code></a>, <a class="link" href="../../reference/io_context.html" title="io_context"><code class="computeroutput"><span class="identifier">io_context</span></code></a>, or a <a class="link" href="../../reference/Handler.html" title="Handlers">completion
          handler</a>. This argument determines the context in which the coroutine
          is permitted to execute. For example, a server's per-client object may
          consist of multiple coroutines; they should all run on the same <code class="computeroutput"><span class="identifier">strand</span></code> so that no explicit synchronisation
          is required.
        </p>
<p>
          The second argument is a function object with signature:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">coroutine</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">yield_context</span> <span class="identifier">yield</span><span class="special">);</span>
</pre>
<p>
          that specifies the code to be run as part of the coroutine. The parameter
          <code class="computeroutput"><span class="identifier">yield</span></code> may be passed to
          an asynchronous operation in place of the completion handler, as in:
        </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">length</span> <span class="special">=</span>
  <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
    <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">);</span>
</pre>
<p>
          This starts the asynchronous operation and suspends the coroutine. The
          coroutine will be resumed automatically when the asynchronous operation
          completes.
        </p>
<p>
          Where an asynchronous operation's handler signature has the form:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">result_type</span> <span class="identifier">result</span><span class="special">);</span>
</pre>
<p>
          the initiating function returns the result_type. In the <code class="computeroutput"><span class="identifier">async_read_some</span></code>
          example above, this is <code class="computeroutput"><span class="identifier">size_t</span></code>.
          If the asynchronous operation fails, the <code class="computeroutput"><span class="identifier">error_code</span></code>
          is converted into a <code class="computeroutput"><span class="identifier">system_error</span></code>
          exception and thrown.
        </p>
<p>
          Where a handler signature has the form:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
<p>
          the initiating function returns <code class="computeroutput"><span class="keyword">void</span></code>.
          As above, an error is passed back to the coroutine as a <code class="computeroutput"><span class="identifier">system_error</span></code>
          exception.
        </p>
<p>
          To collect the <code class="computeroutput"><span class="identifier">error_code</span></code>
          from an operation, rather than have it throw an exception, associate the
          output variable with the <code class="computeroutput"><span class="identifier">yield_context</span></code>
          as follows:
        </p>
<pre class="programlisting"><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">length</span> <span class="special">=</span>
  <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span>
    <span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">),</span> <span class="identifier">yield</span><span class="special">[</span><span class="identifier">ec</span><span class="special">]);</span>
</pre>
<p>
          <span class="bold"><strong>Note:</strong></span> if <code class="computeroutput"><span class="identifier">spawn</span><span class="special">()</span></code> is used with a custom completion handler
          of type <code class="computeroutput"><span class="identifier">Handler</span></code>, the function
          object signature is actually:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">coroutine</span><span class="special">(</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">basic_yield_context</span><span class="special">&lt;</span><span class="identifier">Handler</span><span class="special">&gt;</span> <span class="identifier">yield</span><span class="special">);</span>
</pre>
<h6>
<a name="asio.overview.composition.spawn.h0"></a>
          <span><a name="asio.overview.composition.spawn.see_also"></a></span><a class="link" href="spawn.html#asio.overview.composition.spawn.see_also">See
          Also</a>
        </h6>
<p>
          <a class="link" href="../../reference/spawn.html" title="spawn">spawn</a>, <a class="link" href="../../reference/yield_context.html" title="yield_context">yield_context</a>,
          <a class="link" href="../../reference/basic_yield_context.html" title="basic_yield_context">basic_yield_context</a>,
          <a class="link" href="../../examples/cpp03_examples.html#asio.examples.cpp03_examples.spawn">Spawn example (C++03)</a>,
          <a class="link" href="../../examples/cpp11_examples.html#asio.examples.cpp11_examples.spawn">Spawn example (C++11)</a>,
          <a class="link" href="coroutine.html" title="Stackless Coroutines">Stackless Coroutines</a>.
        </p>
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        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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